1. Field of the Invention
The present invention relates to a light beam scanner apparatus which is adapted to successively reflect optical beams each reflecting off a mirror surface of a rotary polygonal mirror to perform scanning.
2. Description of the Related Art
A light beam scanner is conventionally used to reflect light beams emitted from a light beam source such as a laser utilizing a rotating polygonal mirror, thereby performing scanning.
To permit reading or recording of complex images a light beam scanner is required to use more scanning lines to increase resolution. For example, when a light beam scanner is employed in a recording apparatus, the beam diameter needs to be reduced. This permits the use of more scanning lines per frame and increases the number of resolution dots therefor.
In order to increase dots for the resolution within the same scanning time per frame compared to the conventional technique, it may be proposed that the outer diameter of the polygonal mirror is enlarged and the number of each reflecting mirror surface thereof is increased and a motor is driven at the same speed of revolution as the conventional one. There is however a spacial limitation on the outer diameter of the polygonal mirror when considering such an apparatus. In order to obtain high resolution without increasing the outer diameter of the polygonal mirror or retaining the conventional scanning time per frame it is necessary to increase the revolutionary speed of the polygonal mirror.
There are two types of motors, which have been useful in practice. One is a so-called outer rotor type in which its center shaft is fixed and the peripheral elements are rotated and the other is so-called inner rotor type in which its center shaft rotates.
The outer rotor type motor may be used to drive the polygonal mirror in some instances. This is because the deposition of lubricating oils corresponding to oil mist which may leak from a ball bearing provided between the central shaft and the rotatable outer elements on each reflecting mirror surface takes place to a lower degree compared to the latter inner rotor type motor.
With the outer rotor type motor, less reliability is however, expected at high speed rotation compared to the inner rotor type motor. Accordingly, it is preferable to choose an inner rotor type motor for a light beam scanner since it needs to rotate a polygonal mirror at very high speed.
However, when the inner rotor type motor is applied to the light beam scanner, the bearing from which the oil mist may leak approaches each reflection surface of the polygonal mirror. In particular, the high speed rotation of the polygonal mirror facilitates the deposition o, oils on each reflecting mirror surface. As a consequence, an inner rotor type motor brings about inconvenient influence upon the recording of images. A solution of such a problem has been to propose that a lubricating oil anti-deposition plate be provided between the bearing and the polygonal mirror as disclosed in Japanese Utility Model Application Laid Open No. 3513/1981. Such a structure, however, has to increase the number of components, and air resistance increases upon rotation of the polygonal mirror.